The propeller shafts of marine vessels require always a sealing arrangement for preventing the sea or fresh water from entering into the interior of the stern tube or thruster where at least the bearings of the shaft and often also some kind of a gear are located. Very often the bearings of the propeller shaft are arranged in a closed cavity, which is at least partially filled with oil, whereby the seals are also needed for preventing the lubrication oil from leaking to the sea or lake. Thus the minimum requirement is that there is one seal facing outward and one seal facing inward, However, very often there are, for safety reasons, more than one seal, on the one hand, for preventing the entrance of the sea or fresh water, and on the other hand, for preventing the escape of lubrication oil.
FIG. 1 illustrates a state-of-the-art propeller shaft seal based on lip type sealing rings. The Figure shows that several sealing rings are applied for the sealing, i.e. there are two left hand side rings for preventing the entrance of seawater inside the thruster and two right hand side rings for preventing the lubrication oil from leaking to the sea. A problem with this type of sealing is the water pressure acting on the outermost sealing ring. The pressure is, naturally, the higher; the deeper is the propeller shaft. Therefore it is common practice to apply pressure between each set of lip type seals. By means of the pressure inside the seal between the sealing rings the force by which the seal lip is pressed against the shaft or shaft sleeve is regulated in view of sealing, wear and energy consumption. The correct pressure is normally assured by means of placing a header or monitoring tank, which contains oil, at an appropriate height above the seal. In other words, it is the hydrostatic oil pressure that forms the desired counter pressure.
However, in cases when the draught of the vessel is reduced or has to be reduced, for some reason, the propeller shaft is raised closer to the waterline. This results in the reduction of the water pressure against the seal, whereby the constant oil pressure on the opposite side of the sealing ring easily exceeds the water pressure. An almost unavoidable consequence is an oil leakage to the sea or lake. Situations where such an oil leakage may take place relate, at least, to cases when a marine vessel is brought to a dry dock for inspection, maintenance and/or repair. Also, the type and/or the construction of the vessel may be such that the draught of the vessel is relatively low, and may also vary depending on the operating conditions. In other words, sometimes the construction of the vessel and its propulsion arrangement may be such that the header or monitoring tank has to be positioned at a height above the waterline, whereby the hydrostatic oil pressure is relatively high.
If the seal in question is applied in a thruster the problem is even more serious. The placement of the header or monitoring tank down within the thruster is not an option since the tank would no longer be accessible for filling or monitoring. The monitoring tank is therefore placed up within the vessel frame. As a result the header or seal monitoring tank is placed at a distance above the propeller shaft the distance corresponding at least to the height of the thruster. When using such thrusters in vessels with a low draught there occurs in succession such situations that the assurance of the correct pressure within the seal is impossible.
A thruster as here understood is such a propulsion device of a marine vessel that is formed of at least a propeller unit situated, at least in its operating position, beneath the hull of the marine vessel. The thruster may be a steerable one, a retractable one or a stationary one. The drive of the propeller may be arranged mechanically, hydraulically or electrically.